Birefringent films comprising polymerised liquid crystal (LC) material are known in prior art. For example, GB 2 324 382, GB 2 330 360 and GB 2 357 061 disclose films comprising polymerised or crosslinked nematic or cholesteric LC material with planar, tilted, splayed or homeotropic structure and macroscopically uniform orientation.
Polymerised LC films are usually prepared by dissolving a mixture of two or more polymerisable LC compounds in an organic solvent and coating the solution onto a substrate. The solvent is allowed to evaporate and the remaining LC material is quickly polymerised by exposure to UV radiation while it is still in its LC phase. The polymerisation fixes the oriented structure and consequently the anisotropical properties of the material. It is also possible for example to prepare cholesteric liquid crystal (CLC) pigment flakes from a polymerised CLC film by separating the film from the substrate and grinding it to give small flakes, as described in WO 97/30136.
By using printing techniques it is possible to cover discrete areas of a substrate with an LC material to form a pattern, or to directly prepare LC pigments of a specific size and shape.
For example, WO 97/30136 discloses a specific method, i.e., gravure printing, in the shape of small droplets of a specific size onto a substrate, a chiral polymerizable mesogenic material, to form pigment flakes.
WO 96/02597 describes for example a process for printing substrates with a polymerisable CLC material.
However, LC materials often have a high viscosity and high surface tension, which impairs their printing behaviour and leads to wetting problems, undesired structure formation and difficulties in achieving uniform alignment in the printed areas. The LC materials disclosed for example in WO 96/02597 therefore additionally comprise dispersion auxiliaries. However, the use of additives like dispersion auxiliaries increases the material costs and can also negatively affect the optical properties of the LC material.
On the other hand, if the viscosity of the LC material is too low problems can arise when trying to print different LC materials alongside each other with good resolution. For example, if organic solutions of LC materials are used for this purpose it is possible that the low viscosity of the solution causes the LC materials to mix, thereby ruining the desired pattern.
It was therefore an aim of the present invention to provide a polymerisable LC material that is suitable as a system for the preparation of printed polymer films, coatings and layers and does not have the drawbacks of the prior art materials. The LC material should have an LC phase at room temperature, be suited for printing without the need for high temperatures and without the use of modifiers, thinners, dispersion agents, polymerisable binders or monomer compounds that can be converted into a polymer binder by polymerisation or solvents, and should help to form the required structure necessary to achieve specific optical effects in LC polymer films. In particular, the LC material should have a suitable viscosity, which is high enough to enable printing with high resolution and low enough to allow good wetting of the substrate and alignment and avoid undesired structure formation.
A further aim of this invention relates to a method of preparing polymers, in particular oriented polymer films, patterns, images and pigments from a printable, polymerisable LC material according to this invention, which allows a fast, reliable and inexpensive fabrication.
A further aim of this invention relates to the use of nematic liquid crystal mixtures as printable systems for the preparation of polymer films, markings and pigment flakes.
A further aim of this invention relates to the use of chiral nematic liquid crystal mixtures as printable systems for the preparation of opticaly variable polymer films, markings and pigment flakes.
A further aim of this invention is to provide an advantageous use of the LC materials, polymers and pigments according to this invention, in particular in optical, electrooptical, electronic, semiconducting, decorative and security applications.
A further aim of this invention is the advantageous use of chiral nematic liquid crystal mixtures according to this invention especially for use on paper and other porous substrates.
Further aims of this invention relate to optical, electrooptical, electronic, semiconducting, decorative, security, authentification and identification markings or devices comprising an LC material, polymer or pigment according to this invention.
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors have found that the above aims can be achieved by providing a polymerisable LC material as described below.
Definition of Terms
In connection with liquid crystal layers and films as described in the present application, the following definitions of terms as used throughout this application are given.
The term ‘film’ as used in this application includes self-supporting, i.e. free-standing, films or foils that show more or less pronounced mechanical stability and flexibility, as well as precoated, preprinted or laminated foils wherein the coating or printing can be partial or complete, as well as coatings or layers on a supporting substrate or between two or more substrates.
The term ‘marking’ includes films or coatings or layers covering the entire area of a substrate, as well as markings covering discrete regions of a substrate for example in the shape of a regular pattern or image.
The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal or mesogenic compound’ should denote materials or compounds comprising one or more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e., groups with the ability to induce liquid crystal phase behaviour. Liquid crystal compounds with rod-shaped or board-shaped groups are also known in the art as ‘calamitic’ liquid crystals. Liquid crystal compounds with a disk-shaped group are also known in the art as ‘discotic’ liquid crystals. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit a liquid crystal phase themselves. It is also possible that they show liquid crystal phase behaviour only in mixtures with other compounds, or when the mesogenic compounds or materials or the mixtures thereof are polymerized.
The term ‘chiral nematic’ means a liquid crystal material in which the director direction varies monotonuously through the film creating a helical structure.
For the sake of simplicity, the term ‘liquid crystal material’ is used hereinafter for both liquid crystal materials and mesogenic materials, and the term ‘mesogen’ is used for the mesogenic groups of the material.
The director means the preferred orientation direction of the long molecular axes (in case of calamitic compounds) or short molecular axis (in case of discotic compounds) of the mesogens in a liquid crystal material.
The term ‘planar structure’, ‘planar alignment’ or ‘planar orientation’ refers to a layer or film of liquid crystal material wherein the director is,substantially parallel to the plane of the film or layer.
The term ‘homeotropic structure’, ‘homeotropic alignment’ or ‘homeotropic orientation’ refers to a layer or film of liquid crystal material wherein the director is substantially perpendicular to the film plane, i.e., substantially parallel to the film normal.
The term ‘tilted structure’, ‘tilted alignment’ or ‘tilted orientation’ refers to a layer or film of liquid crystal material wherein the director is tilted at an angle θ of between 0 and 90 degrees relative to the film plane.
The term ‘splayed structure’, ‘splayed alignment’ or ‘splayed orientation’ means a tilted orientation as defined above, wherein the tilt angle varies monotonuously in the range from 0 to 90°, preferably from a minimum to a maximum value, in a direction perpendicular to the film plane.
For sake of simplicity, a film comprising liquid crystal material with a planar, homeotropic, tilted or splayed orientation, alignment or structure is hereinafter also referred to as ‘planar film’, ‘homeotropic film’, ‘tilted film’ and ‘splayed film’, respectively.
The term “reflective substrate” covers substrates with mirrorlike surfaces for printing onto metal films, substrates showing Lambertian reflection, which are especially suitable when printing onto, for example, pearlescent pigment systems, and substrates that comprise or are part of an optically variable device (OVD), like, for example, a diffraction grating, hologram or kinegram.
“Reflection” means reflection of light inside the visible range of the spectrum (with wavelegnths from approximately 400 to 800 nm) and outside the visible range, e.g. in the UV or IR range (with wavelengths of less than 400 nm or more than 800 nm).